By Bob Berwyn, InsideClimate News

Sea level rise is accelerating, and natural and man-made barriers block the ability of many wetlands on the U.S. West Coast to move inland with the tide. Credit: Tracie Hall/CC-BY-SA-2.0

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Rising seas will drown most wetlands on the U.S. West Coast in less than a century, a new study led by the U.S. Geological Survey warns. In many areas, the wetlands won't be able to migrate inland without help.

"Wetlands provide flood protection and sequester carbon. They filter water and improve water quality, they exchange nutrients with the ocean," said Karen Thorne, a U.S. Geological Survey sea level rise researcher who led the research, published this week in the journal Science Advances. Keeping them healthy is critical for those reasons and for coastal wildlife, since they serve as a nursery and feeding area for fish and marine animals, she said.

In many areas, however, the endangered ecosystems won't be able to ride the rising tides inland because of natural barriers and man-made obstacles like sea walls that stand in the way.

Helping them to survive will require political and public support for reallocating land to let wetlands shift and expand, and even abandoning some developed areas, the study's authors write.

Sea level rise is accelerating as rising global temperatures cause the oceans to expand and glaciers and polar ice sheets to melt. If greenhouse gas pollution isn't cut to near zero, the oceans could be rising 4 inches per decade by 2100, scientists reported in another study, published last week. Several recent studies suggest that some of Antarctica's ice sheets could collapse much faster than expected, which would speed up sea level rise well beyond the rates used in the wetlands study.

The study found that under moderate to high sea level rise projections of 2 to 3 feet by 2110, California, Washington and Oregon would lose at least 83 percent of their existing coastal wetlands.

Some of the most ecologically and economically valuable wetland types in California and Oregon would be 100 percent wiped out by 2110 without extensive and immediate mitigation and adaptation measures, it found.

These Wetlands Will Need Help to Survive

Wetlands are dynamic systems that can shift and adapt to at least some sea level rise on their own, but in many West Coast locations, the coastal marshes are bordered by natural or manmade features like cliffs, seawalls and other infrastructure that prevent them from moving inland as the water rises. Along the Atlantic Coast and the Gulf of Mexico, by contrast, there are "greater opportunities for wetlands to migrate inland," the authors wrote.

The scientists identified the potential to make wetlands more resilient with "nature-based" adaptation measures, like bolstering the replenishment of organic materials like ocean and river sediments that build up wetlands. The study notes that dams and other water diversion in the region have cut off the natural supply of sediment in several of the wetlands.

At sites with little natural replenishment and low migration potential, costly engineered solutions may be needed—or communities will need to prepare for "accepting tidal wetland loss later in the century," the study concluded.

The research team included state, federal and university scientists who looked at 14 coastal wetlands from Seattle to Tijuana.

The sampled areas represent a major portion of the flyway for birds, which use the wetlands as feeding stops on long migrations between North and South America.

California Explores Building Natural Resilience

Overall, sea level rise threatens about $100 billion worth of coastal property in California alone, with billions more dollars at stake in Oregon and Washington. But bolstering coastal wetlands resiliency to climate change could reduce those damages significantly, The Nature Conservancy concluded in a recent report.

In California, a team of experts recently compiled a series of case studies of areas that are already being affected by sea level rise. It assessed innovative approaches, such as upland restoration for habitat migration, adding sediment to build up the elevation, and building dunes to prevent floods from high tides and storms. In many cases, they found, the best options for protecting coastal communities, along with wetlands, may be to boost natural infrastructure like coastal dunes and floodplains rather than pursuing costly artificial hardening measures like seawalls, dikes and levees.

"We're trying to open peoples' minds to alternatives to hardening shorelines," said Jenna Judge, a coordinator with the NOAA Sentinel Site program, a project team leader for the report.

Long-time coastal engineer Bob Battalio, who was involved in one of the case studies, said "one of the big impediments is fear, the concern that if you realign landward, the ocean is immediately going to chase you right back, and then you've lost something."

"We think we can control everything and just build a wall, right? But when you build something in an area where waves are active, there's a negative feedback on the system," Battalio said.

"I've come to the conclusion that engineering isn't really the issue," he said. "It's more of a governance and social justice problem."